lcnt

A runtime system Lock Profiling tool.

The lcnt module is used to profile the internal ethread locks in the
Erlang Runtime System. With lcnt enabled, Internal counters in the
runtime system are updated each time a lock is taken. The counters stores
information about the number of acquisition tries and the number of collisions
that has occurred during the acquisition tries. The counters also record the
waiting time a lock has caused for a blocked thread when a collision has occurred.

The data produced by the lock counters will give an estimate on how well
the runtime system will behave from a parallelizable view point for the
scenarios tested. This tool was mainly developed to help erlang runtime
developers iron out potential and generic bottlenecks.

Locks in the emulator are named after what type of resource they protect and where
in the emulator they are initialized, those are lock 'classes'. Most of those
locks are also instantiated several times, and given unique identifiers, to increase
locking granularity. Typically an instantiated lock protects a disjunct set of
the resource, i.e ets-tables, processes or ports. In other cases it protects a
specific range of a resource, e.g. pix_lock which protects index to process
mappings, and is given a unique number within the class. A unique lock in lcnt
is referenced by a name (class) and an identifier, {Name, Id}.

Some locks in the system are static and protects global resources, for example
bif_timers and the run_queue locks. Other locks are dynamic and not
necessarily long lived, for example process locks and ets-table locks. The
statistics data from short lived locks can be stored separately when the locks
are deleted. This behavior is by default turned off to save memory but can be
turned on via lcnt:rt_opt({copy_save, true}). The lcnt:apply/1,2,3
functions enables this behavior during profiling.

start() -> {ok, Pid} | {error, {already_started, Pid}}

Starts the lock profiler server. The server only act as a medium for the
user and performs filtering and printing of data collected by lcnt:collect/1.

stop() -> ok

Stops the lock profiler server.

collect() -> ok

Same as collect(node()).

collect(Node) -> ok

Node = node()

Collects lock statistics from the runtime system. The function starts a
server if it is not already started. It then populates the server with lock
statistics. If the server held any lock statistics data before the collect then
that data is lost.

Note!

When collection occurs the runtime system transitions to a single thread,
blocking all other threads. No other tasks will be scheduled during this
operation. Depending on the size of the data this might take a long time
(several seconds) and cause timeouts in the system.

clear() -> ok

Same as clear(node()).

clear(Node) -> ok

Node = node()

Clears the internal lock statistics from the runtime system. This does not clear the
data on the server only on runtime system. All counters for static locks are zeroed,
all dynamic locks currently alive are zeroed and all saved locks now destroyed are removed.
It also resets the duration timer.

Lock Name and Id for ports and processes are interchangeable with the use of lcnt:swap_pid_keys/0 and is the reason why pid() and port() options can be used in both Name and Id space. Both pids and ports are special identifiers with stripped creation and can be recreated with lcnt:pid/2,3 and lcnt:port/1,2.

Option description:

{combine, bool()}

Combine the statistics from different instances of a lock class.
Default: true

{locations, bool()}

Print the statistics by source file and line numbers.
Default: false

{max_locks, MaxLocks}

Maximum number of locks printed or no limit with none.
Default: 20

{print, PrintOptions}

Printing options:

name

Named lock or named set of locks (classes). The same name used for initializing the lock in the VM.

id

Internal id for set of locks, not always unique. This could be table name for ets tables (db_tab), port id for ports, integer identifiers for allocators, etc.

type

Type of lock: rw_mutex, mutex, spinlock, rw_spinlock or proclock.

entry

In combination with {locations, true} this option prints the lock operations source file and line number entry-points along with statistics for each entry.

tries

Number of acquisitions of this lock.

colls

Number of collisions when a thread tried to acquire this lock. This is when a trylock is EBUSY, a write try on read held rw_lock, a try read on write held rw_lock, a thread tries to lock an already locked lock. (Internal states supervises this).

ratio

The ratio between the number of collisions and the number of tries (acquisitions) in percentage.

time

Accumulated waiting time for this lock. This could be greater than actual wall clock time, it is accumulated for all threads. Trylock conflicts does not accumulate time.

duration

Percentage of accumulated waiting time of wall clock time. This percentage can be higher than 100% since accumulated time is from all threads.Default: [name,id,tries,colls,ratio,time,duration]

save(Filename) -> ok

Convenience functions

apply(Fun) -> term()

Same as apply(Fun, []).

apply(Fun, Args) -> term()

Fun = fun()

Args = [term()]

Clears the lock counters and then setups the instrumentation to save all destroyed locks.
After setup the fun is called, passing the elements in Args as arguments.
When the fun returns the statistics are immediately collected to the server. After the
collection the instrumentation is returned to its previous behavior.
The result of the applied fun is returned.

apply(Module, Function, Args) -> term()

Module = atom()

Function = atom()

Args = [term()]

Clears the lock counters and then setups the instrumentation to save all destroyed locks.
After setup the function is called, passing the elements in Args as arguments.
When the function returns the statistics are immediately collected to the server. After the
collection the instrumentation is returned to its previous behavior.
The result of the applied function is returned.

pid(Id, Serial) -> pid()

Same as pid(node(), Id, Serial).

pid(Node, Id, Serial) -> pid()

Node = node()

Id = integer()

Serial = integer()

Creates a process id with creation 0. Example:

port(Id) -> port()

Same as port(node(), Id).

port(Node, Id) -> port()

Node = node()

Id = integer()

Creates a port id with creation 0.

Internal runtime lock counter controllers

The following functions control the behavior of the internal counters.